The present invention relates to shift control apparatus and method for a continuously-variable transmission mounted in a vehicle.
When carrying out a shift control for a continuously-variable transmission mounted in a vehicle, in general, a target speed ratio (i.e., transmission ratio) is determined from a target rotational speed of an input shaft according to an operating condition of the vehicle by using a map. Such a map represents a shift pattern according to a vehicle speed and an accelerator operation quantity (i.e., accelerator manipulated variable), and has been stored as a map of the target input-shaft rotational speed. In such a case, the map represents a shift (pattern) line every range of the accelerator operation quantity, so as to produce a favorable acceleration of the vehicle at the time of vehicle start. Hence, a down-shift amount becomes excessive when reaccelerating the vehicle, e.g., at the time of a kickdown operation. Accordingly at this time, the engine speed excessively increases, and a time lag in the production of driver's expected (desired) acceleration is caused.
As a countermeasure against such a problem, it is conceivable that a variation of speed ratio is made to be suppressed at the time of a request of high acceleration. A Japanese Patent No. 2593432 exemplifies a previously proposed technique for controlling a continuously variable transmission. In this technique, the speed ratio is fixed at a constant value when a throttle opening degree becomes greater than or equal to a threshold value. In this technique, the fixed constant value of speed ratio is determined to take on a speed ratio value of the time when the throttle opening reaches the threshold value, shown on a shift map. Thus by fixing the speed ratio so as not to enlarge the speed ratio, the increase of engine speed produces an increase of a driving force of vehicle quickly (immediately). Thereby, the time lag between a time when the throttle opening increases and a time when the driver obtains an acceleration feeling is shortened. Accordingly, an uncomfortable feeling is reduced, and a feeling responsiveness of acceleration is enhanced.
However, in the technique disclosed in the above Japanese Patent, the speed ratio is kept at the constant value unless the throttle opening returns below than the threshold value and its hysteresis setting. Namely, the speed ratio does not vary, even after the initial acceleration request of the driver has been satisfied. This causes the driver to feel uncomfortable (i.e., feel odd). Therefore, in order to resolve this uncomfortable feeling, a Japanese Patent Application Publication No. 2004-183854 exemplifies a previously proposed technique; which is constructed for providing the increase of vehicle speed according to the driver's acceleration request, even at the time of vehicle reacceleration such as the kickdown acceleration, in the similar manner as at the time of a vehicle acceleration from standstill.
This technique is configured to switch between a map shift mode and a map-less shift mode. In the map shift mode, the speed ratio is variably controlled by tracing a shift map prepared so as to relate the speed ratio to the vehicle speed and the accelerator operation quantity. In the map-less shift mode, the shift is carried out by means of a map-less shift control which does not trace the shift map, when a predetermined condition related to acceleration is satisfied (reacceleration such as the kickdown operation).